sensors/zones: place in namespace and cleanup

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I527dbc8477a232945f696227a7b0b2adbee45175
diff --git a/interfaces.hpp b/interfaces.hpp
index 2002a61..b4b236b 100644
--- a/interfaces.hpp
+++ b/interfaces.hpp
@@ -2,6 +2,9 @@
 
 #include <chrono>
 
+namespace pid_control
+{
+
 struct ReadReturn
 {
     double value;
@@ -66,3 +69,5 @@
     int64_t _min;
     int64_t _max;
 };
+
+} // namespace pid_control